Okay...there is a standard and reverse rotation. The reverse rotation has a different cam. The crank has the hatch marks for oiling the rear seal in a reverse pattern. The hatch marks or
oil grooves can be ground smooth to use the crank if not the rear seal will leak. I think the
oil pump and water pump are setup for the reverse rotation also. The heads are low riser type. The crank and rods are std 390 type. The marine blocks are very good if not corroded by salt water. They are cast as a sideoiler but drilled for centeroiler applications. It's really not advised to try to drill it for sideoiler use as some have the sideoiler hump ground down a bit to clear the marine motor mounts and it's a MAJOR PIA to drill. If you are only looking to purchase one get the standard rotation engine. Depending on how and what you will use the engine for you might only end up using the block. If your doing a budget build look into them as you can use most of the engine to rebuild for a nice stock runner. Upgrade the cam and valve train also. So it really boils down to what you want to build and want to spend. $1000 to $1500 each is the going price I see. Check the block for overbore and feel out the seller for info. Are they in running condition in a boat....how many rebuilds? etc..... Let me know what other info your looking for. I can give you some part numbers if needed to verify things. Email me directly if you like. Glenn.